A Stirling engine is a type of heat engine that operates based upon the expansion and contraction of a working gas (e.g., air, helium, hydrogen, etc.) contained in side the working spaces of the engine, where the working spaces comprise a hot workspace and a cold workspace, with a regenerator between them. As the working gas moves into the hot space, it expands, diving the pistons apart, it is then transferred to the cold space where it contracts and fly wheel energy is used to further compress it. The working gas is then transferred back to the hot mace and the cycle repeats.
Numerous Stirling engine designs have been developed and recently interest in Stirling engines has been renewed owing to such factors as the ability to power a Stirling engine with a renewable energy source(s), their quiet operation, and low vibration characteristics.
Three primary configurations commonly known as alpha, beta and gamma Stirling engines have been developed. While many improvements have been made regarding the efficiency of such engines substantial research and development is ongoing to address such concerns as size, cost, power, torque, variable speed, etc.